Conventionally, as a road that regulates entrance of vehicles, a carpool lane has been known which is employed by a road system mainly seen in large cities in North America. The carpool lane, which is also called an HOV lane, is a lane that allows only vehicles with two or more passengers to travel. As an example of the carpool lane, a lane provided on a highway, a lane that short-cuts an interchange and the like are known. A road system that employs the carpool lane encourages a plurality of persons to get on the same vehicle by giving them preferential treatment that they can arrive at a destination in a shorter time by driving along the carpool lane, thereby trying to reduce a volume of traffic to alleviate traffic congestion.
As a technique for guiding along the carpool lane, Patent Document 1 discloses a navigation system that enables driving along a lane efficiently which limits changing to or from other lanes. The navigation system reads from a DVD map data containing road information on an entrance/exit point on an express highway provided with a carpool lane, and stores in a data buffer. Using the map data stored in the data buffer, a route search processing unit performs route search processing considering whether the carpool lane can be used. To carry out route guidance using the carpool lane, a carpool lane guiding unit gives prescribed lane change information using images and voices at timing when an entrance/exit point at which the route is to be changed comes within a prescribed distance from the position of the vehicle.